A known first-second gear shift valve predominant over the shifting between the first and second gear ratios of the transmission mechanism of an automatic power transmission usually comprises a valve spool slidable in a valve chamber and formed with pressure acting areas to be respectovely acted upon by a governor pressure variable with vehicle speed and a throttle pressure variable with the opening degree of the carburetor throttle valve of the engine. The pressure actimg areas are arranged so that the forces resulting from the governor and throttle pressures acting thereon are exerted in opposite directions. When the valve spool thus arranged is held in a downshift position producing the first gear position in the transmission mechanism, the valve spool cuts off the fluid circuit in which is to be developed a fluid pressure to actuate a fluid operated frictional unit contributing to the shifting to the second gear ratio. When the valve is moved from the downshift position to an upshift position to produce the second gear ratio in the transmission mechanism, the valve spool interrupts the communication by which the fluid pressure to actuate a frictional unit contributive to the shifting to the first gear ratio is to be passed to the frictional unit.
A first-second gear shift valve commonly in use further has a function that, when a manual first forward drive range position is selected at the manually operated gear shift valve and a signal fluid pressure representative of such a requirement is passed to the shift valve, the valve spool of the shift valve is moved into the downshift position and causes the corresponding frinctional unit to maintain the first gear ratio. A conventional shift valve is provided with two ports to be supplied with this signal fluid pressure so that the signal fluid pressure developed in one of the ports serves to force the valve spool to the downshift position thereof and the signal fluid pressure developed in the other port is passed through the shift valve to the frictional unit to be put into operation to hold the first gear ratio in the transmission mechanism. Formation of such two ports used for different purposes inevitably results in elongation of the shift valve.
On the other hand, a first-second gear shift valve is supplied with an additional fluid pressure when kickdown condition is brought about with the accelerator pedal of the vehicle depressed a diatance greater than a predetermined value. Such an additional fluid pressure acts on the valve spool of the gear shift valve against the force resulting from the governor pressure also acting on the valve spool so that the shift point between the first and second gear ratios is made higher with respect to the vehicle speed. In this instance, it is desirable that the shift points for the upshifting and downshifting under kickdown condition can be determined independently of each other so that both of the shift points for the upshifting and downshifting are fully acceptable. A shift valve providing such features may be realized if the valve spool is formed with first, second and third circumferential lands which are smaller in cross sectional area in a direction coincident with the direction in which the valve spool is to be moved when in making an upshift. A fluid pressure to be developed under kickdown condition is applied to the differential pressure acting area between one pair of circumferential lands of the valve spool in the upshift position and to the differential pressure acting area between another pair of circumferential lands of the valve spool in the downshift position so that the valve spool is urged to move against the governor pressure by different forces depending upon in whichsoever of the positions the valve spool is when the kickdown condition is produced. If, in this instance, the differential pressure acting area between the first and second circumferential lands and the differential pressure acting area between the first and third circumferential lands are acted upon by the kickdown fluid pressure when the valve spool is in the downshift and upshift positions, respectively, not only a drain port which must be provided adjacent to the upshifting end of the valve spool for accommodating the axial movement of the valve spool but an additional drain port must to provided through which the differential pressure acting area between the second and third circumferential lands of the valve spool is to be made open when the valve spool is moved to the downshift position.
An object of the present invention is, therefore, to provide a first-second gear shift valve in which the signal fluid pressure representative of the manual first forward drive position selected at the manually operated gear shift lever is introduced into one and the same port utilized not only for applying the fluid pressure onto the pressure acting area of the valve spool through the port but for passing the fluid pressure through the shift valve by way of the particular port.
Another object of the present invention is to provide a first-second gear shift valve having first, second and third circumferential lands which are arranged in such a manner that the third circumferential land is constantly ready to be acted upon by a fluid pressure to be developed under kickdown condition and that the differential pressure acting areas formed between the three circumferential lands are utilized to be acted upon by different fluid presdepending upon the upshift and downshift positions assumed by the valve spool so that there is no need for the provision of any extra drain port other than the drain port which must be provided adjacent the upshifting end of the valve spool.